1. Field of the Invention
The present invention relates to a process for the production of a sintered silicon nitride made article having a high strength.
2. Description of the Related Art
Generally, strength of a ceramic material depends on, for example, a porosity ratio, a particle size and a surface condition of a crystal. Such factors affect strength of a sintered article made of silicon nitride which is one of important ceramic materials used for structural members.
In order to increase the strength of a Si.sub.3 N.sub.4 based sintered article, sintering aids and sintering processes have been developed. For example, Am. Ceram. Soc. Bull., 52, 560, (1973) reports that a bending strength of up to 100 Kg/mm.sup.2 with the use of a hot press sintering, and Preprint for the 1981 Annual Conference of the Ceramic Association of Japan, 178 (1981) reports that the same strength as above with the use of a low pressure sintering. In each case, the strength is increased by extremely reducing the porosity.
Japanese Patent Kokoku Publication Nos. 21091/1974 and 38448/1973 describe a process for the production of the silicon nitride made sintered article based on Si.sub.3 N.sub.4 --Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3 in which Y.sub.2 O.sub.3 is used as a main sintering aid.
As described in these Publications, it may be contemplated thought that the strength and toughness of the ceramic material are increased since Si.sub.3 N.sub.4 in a .beta.-crystal lattice is present in a fibrous structure and dispersed in a matrix phase. Namely, the lattice of .beta.-crystal is a hexagonal one, and an anisotropical crystal growth along C axis is effectively used. A fibrous particle of .beta.-Si.sub.3 N.sub.4 crystal may grow to more than ten and few microns along a longitudinal axis as described in Japanese Patent Kokoku Publication No. 38448/1973 and J. Ceram. Ass. Jpn., 94, 96 (1986).
Since the growth of the fibrous structure is required in the prior arts as described above, abnormal growth of the crystal particles and generation of pores in relation to such growth may arise. Thus, this growth is not necessarily effective to increase the strength of the ceramic material.
In addition, in the case where the hot press sintering is used in order to form the fibrous structure as described in Japanese Patent Kokoku Publication No. 21091/1974 or where a raw material of the .beta.-crystal of Si.sub.3 N.sub.4 which has been thermally treated for the fibrous structure growth is added, the increase of the strength cannot be expected.